In fluid networks, it is common to distribute fluid from a source to one or more points of consumption (loads). In order to provide the correct amount of fluid under varying demands, one or more control valves are commonly provided. These control valves respond to a control signal to create variable restrictions in the system providing an appropriate amount of fluid to each load. For example, a control signal might be supplied by a thermostat, and a control valve might respond by changing the flow of heating or cooling fluid through a heat exchanger. If the control valve is chosen with a maximum opening that is larger than the maximum needed for the application, then it must be controlled to close excessively at all times. This excessive closure results in unstable control as the control valve changes from an open condition to a closed condition repeatedly rather than settling at the proper location. Conversely, if the valve is chosen with too small a maximum opening, excessive pumping energy is required to address unnecessary pressure drops arising in the system. This problem is compounded by the fact that control valves are ordinarily available only in fixed steps, forcing the user to select one or another type of error.
The known systems generally have differing amounts of surplus pressure at different terminals. An ordinary control valve provides no means for reading the flow rate of the fluid, nor of manually adjusting its maximum opening which will cause an ordinary control valve to incorrectly control the flow of fluid. While the amount of surplus pressure might be calculated in theory, in practice the calculations are often not done due to their complexity, or are inaccurate due to construction variations. This problem is frequently addressed by installing balancing valves, which provide a calibrated adjustable restriction and a means of measuring the flow rate. A balancing contractor is then employed to adjust these balancing valves throughout the system so that at maximum flow conditions all terminals receive the correct flow of fluid without excess. Further, in some systems the pressure at each terminal can change as the loading on the system causes the system resistance to change and as the pumping power is altered to correspond to changing loads. The result can be that under different load conditions, the system uses more power than necessary, or some terminals do not get the amount of fluid they need, or the operation of some terminals is unstable. To correct this, pressure controllers are sometimes incorporated either as separate components or integrated with a control valve.
The known valves suffer from the problem that the restriction created by the balancing valve is not taken into account by the control valve, so that a portion of the control valve's stroke is wasted.
Some prior devices combine the function of a control valve and a balancing valve in a single unit, providing improved performance of the combined unit. With the control function and balancing accomplished by a single device, it is possible to provide improved control performance tailored to the exact conditions experienced at a given terminal.
Some prior devices combine a control valve in the same housing with a pressure compensator to make a pressure independent control valve. These devices in some cases also include an adjustment for their maximum flow.
A problem suffered by some prior art devices is that they are bulky, especially in large sizes.
A further problem suffered by prior art devices is that they do not have the correct relationship between their stem position and the heat transfer of the connected device, particularly when they are adjusted to a particular maximum flow.
The present invention seeks to provide a device which addresses one or more of the problems presented by prior art arrangements.